1. Field of the Invention
The invention relates generally to compositions and methods of reducing corrosion in cement exposed to carbon dioxide.
2. Description of the Related Art
Cement, such as Portland cement, is ubiquitous in wells for sealing the well casing to the formation, isolating zones, plugging old wells, squeeze cementing treatments, cementing of steam producing or injection wells, disposal wells, and permafrost environments, remediating failed cement, and many other uses. Cement is also used in many civil engineering projects such as roads, bridges, dams, and buildings. In all of these applications, cement can suffer corrosion due to acidic contaminants or direct contact with acidic liquid, particularly wet carbon dioxide (CO2). When carbon dioxide dissolves in water, carbonic acid is formed. This acid then attacks the calcium hydroxide found in the cement, resulting in corrosion and failure of the cement. This issue is especially problematic in sour wells (producing CO2 and H2S containing oil and gas) and CO2 injection wells. CO2 injection is a process used to enhance crude oil recovery and more recently, is a possible method of sequestering greenhouse gases in underground aquifers.
When carbon dioxide is exposed to water, carbonic acid (H2CO3) is formed using the following equation: CO2+H2O→H2CO3. Carbonic acid can then attack calcium hydroxide that is a major component present in cement compositions through the following reaction: H2CO3+Ca(OH)2→CaCO3+2.H2O. As the continuous exposure of the cement to wet CO2 or carbonic acid continues, the calcium carbonate can react with carbonic acid to produce water soluble calcium bicarbonate using the following equation: CaCO3+H2CO3→Ca(HCO3)2, which opens up the channels for further carbonic acid propagation into cement mass and causes further corrosion of the cement.
To alleviate the problems associated with CO2 corrosion, attempts have been made to develop cement compositions that are resistant to CO2 and its effects. If a well is cemented with CO2 resistant cement during well construction, it will have extended life when it is used either as a CO2 injection well or as a CO2 producing well. In one such example, the cement has some or all of the Portland cement replaced by fly ash and/or calcium aluminate. Such cements have been used in petroleum and geothermal wells ranging in temperature from 140° F. to 300° F. and have been evaluated in laboratory experiments up to 700° F. Under such conditions, regular Portland cement suffers large amounts of corrosion. These fly ash/calcium aluminate cements are somewhat effective at reducing corrosion, but they are more expensive and difficult to apply due to their sensitivity to contamination. Calcium aluminate cement will flash set if it comes into contact with any normal Portland cement. This prevents the same equipment from being used to pump both Portland and calcium aluminate cements, a whole separate fleet of pumping equipment must be retained.
A need exists for methods and compositions useful for reducing corrosion in cement applications, such as in wet CO2 environments. It would be useful if the additives were compatible with all types of cements to enable the same equipment to be used when handling cement compositions without the corrosion inhibitors.